Method and apparatus for simultaneous observation of three-degrees of vibrational freedom using single heterodyne beam

ABSTRACT

A laboratory system has demonstrated the measurement of three degrees of vibrational freedom simultaneously using a single beam through heterodyne speckle imaging. The random interference pattern generated by the illumination of a rough surface with coherent light can be exploited to extract information about the surface motion. The optical speckle pattern is heterodyne mixed with a coherent reference. The recorded optical data is then processed to extract three dimensions of surface motion. Axial velocity is measured by demodulating the received time-varying intensity of high amplitude pixels. Tilt, a gradient of surface velocity, is calculated by measuring speckle translation following reconstruction of the speckle pattern from the mixed signal.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America.

FIELD OF THE INVENTION

The present invention relates to coherent optical vibration sensing,specifically for observation of three-degrees of freedom using temporalheterodyne speckle imaging.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE CO-INVENTORS

An article co-authored by James Perea and Brad Libbey, (theco-inventors), entitled, “Development of a heterodyne speckle imager tomeasure 3-degrees of vibrational freedom,” was submitted to The OpticalSociety earlier in 2016 for publication in an Optical Expresspublication in 2016, the publication date TBD.

BACKGROUND OF THE INVENTION

Coherent optical vibration sensors have been investigated for use innumerous applications including strain measurements, equipmentdiagnostics, medical imaging, and seismic sensing. Various techniquesare utilized to observe surface motion. These techniques include, butare not limited to heterodyne laser Doppler vibrometry (e.g., U.S. Pat.No. 4,834,111 A Khanna et al.) for observation of surface velocity inthe axial dimension of the interrogation beam, shearography (e.g., U.S.Pat. No. 5,011,280 A Hung) for observation of the gradient ofdisplacement in two dimensions, electronic speckle patterninterferometry (e.g., U.S. Pat. No. 4,018,531 A Leendertz) for in-planeor out of plane displacement or out of plane displacement gradients, andspeckle pattern imaging for out of plane displacements. The techniqueslisted are generally used for observation of one or two degrees offreedom. Variations using multiple coherent beams (e.g., U.S. Pat. No.7,242,481 B2 Shpantzer et al.) have been used to observe three degreesof freedom.

It is of interest to simultaneously observe three-degrees of freedomusing a single coherent beam.

The current invention combines elements of heterodyne Doppler vibrometryand digital speckle photography along with signal processing routines tosimultaneously observe velocity in the axial dimension of theinterrogation beam and out of plane tilts of the illuminated region.This provides the ability to observe three-degrees of freedom using asingle coherent beam.

SUMMARY OF THE INVENTION

The disclosure relates to measuring the motion of a surface plane byimaging optical speckle that has been modulated by a heterodyne process.A process takes the sequence of heterodyne images and extracts motionwith three degrees of freedom. The device takes advantage of a probingoptical field that illuminates the rough surface. The spatial opticalfield is described based on previous work. (See, e.g., P. Hariharan,Basics of Interferometry, Academic Press, 2007; P. Jacquot and J. M.Fournier, Interferometry in Speckle Light: Theory and Applications,Springer, 2000; H. J. Tiziani, “Analysis of Mechanical Oscillations bySpeckling,” Applied Optics, vol. 11, no. 12, pp. 2911-2917, 1972; and R.Jones and C. Wykes, “Holographic and Speckle Interferometry,” CambridgeUniversity Press, 1983.) These models are typically independent of time,but this invention takes advantage of surface motion that affects theprobe beam by causing a phase shift due to path length change over time.The invention adds an optical frequency shift to observe these phase andamplitude shifts creating a heterodyne system.

The moving surface alters the path length of a probe beam in time. Toaid in understanding this invention, it is useful to define the geometryof this moving surface. This surface is described in a {ξ,η,z}coordinate system, where ξ and η are in the surface plane and z isorthogonal to the plane. The sensor can determine two types of motion;out of plane pistoning in the axial direction z, and tilting of theplane in two directions. Out of plane motion results in a Doppler phaseshift and tilting results in a translation of the speckle image. (See,e.g., R. Jones and C. Wykes, “Holographic and Speckle Interferometry,”Cambridge University Press, 1983, incorporated herein by reference.) Asingle dimension of tilting θ is presented to simplify illustration,however, the addition of a second tilting degree-of-freedom φ parallelsthe picture below and is superimposed in the section detailing theinvention. The moving surface has a pivot point that is allowed totranslate in the z direction, but is otherwise located at a fixedposition ξ=ξ₀. The position of a scattering point P₁, originally on thez=0 plane at {ξ,0,0}, will be moved to a new location P₂, FIG. 1. Thismotion extends the path length of reflected light; the path length isused to determine the speckle image resulting from these two locationsin a method paralleling Tiziani. (See, e.g., H. J. Tiziani, “Analysis ofMechanical Oscillations by Speckling,” Applied Optics, vol. 11, no. 12,pp. 2911-2917, 1972, incorporated herein by reference.)

In one aspect, an exemplary heterodyne speckle imaging sensor system isdisclosed. In another aspect, an exemplary method for simultaneousobservation of three-degrees of vibrational freedom is disclosed basedon a heterodyne speckle imaging sensor system.

Finally, an exemplary signal processing method is disclosed forcomputing three-degrees of vibrational freedom using a heterodyne beam.Such a signal processing method comprises the steps of accessing acomplete measurement of temporal spatial irradiance data obtained usinga heterodyne beam as stored as pixel frames of image data on a computer;high-pass filtering each pixel independently to isolate heterodynesignal from the complete measurement to output heterodyne information ata local oscillator frequency; applying a computation process forsequential estimates of a detected probe speckle pattern amplitude tocompute two dimensions of tilt based on time sequences; applying aparallel demodulation process based on an arctangent demodulation forDoppler shifted heterodyne signals applied to each pixel to calculatez-axis velocity; and displaying a representation of the computed tiltbased on time sequences and the calculated z-axis velocity on a datadisplay device.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 shows an exemplary incident beam and moving surface geometry usedto describe the three-dimensional vibration sensing method.

FIG. 2 shows an exemplary heterodyne speckle imaging sensor system witha dynamic diffuse-scatterer for the target.

FIG. 3 shows an exemplary method for simultaneous observation ofthree-degrees of vibrational freedom using a heterodyne beam, e.g., of aheterodyne speckle imaging sensor system shown in FIG. 2.

FIG. 4 shows an exemplary process for signal processing a sensorobservation of three-degrees of vibrational freedom using a heterodynebeam as exemplified in FIG. 3.

DETAILED DESCRIPTION

Before entering into the detailed description of one embodiment of thepresent invention according to the accompanying Figures, the theory ofthe present invention will be explained hereinafter.

FIG. 1 shows an exemplary incident beam and moving surface geometry 100used to describe the three-dimensional vibration sensing method. Theobject plane, z=0, is allowed to rotate about a pivot {₀,η,0} andtranslate along the z-axis. An incident electromagnetic beam contacts{ξ_(P),0,0} at angle β before tilting. As the object plane rotates byangle θ, the incident electromagnetic beam impinges on a new spatiallocation, located distance d₁ away along the axis of the beam. The imageplane is located a sufficient distance from the object plane to beconsidered in the far field.

The variation in path length ΔP due to tilt and translation is acombination of the incoming and outgoing light. The incoming light isassumed to be coherent and planar at an angle β relative to the surfaceplane. The image plane is assumed to be sufficiently far such thatoutgoing light is orthogonal to the surface plane. The path variation isgeometrically determined,ΔP=d ₁ +d ₂ −d ₃ −d ₄ΔP=(ξ−ξ₀)[sin(β)(1−cos(θ))+(1+cos(β))sin(θ)]−d ₄(1+cos(β))where θ is angular change due to the tilting surface to which a smallangle assumption is applied sin(θ)≈θ,

${{\cos(\theta)} \cong {1 - \frac{\theta^{2}}{2}}},$and θ²<<θ. This assumption leads to a simplified path length.ΔP≈(ξ−ξ₀)θ(1+cos(β))−d ₄(1+cos(β))

In the description below the direction of illumination, and hence theDoppler velocity, are assumed to be aligned with the z-axis, β=0.ΔP≈(ξ−ξ₀)2 sin(θ)−2d ₄If we maintained the cos(β) term through the subsequent equations, thevelocity would be measured along an axis that is acute with the plane.This increases the utility of the method to horizontal translation aswell as pistoning.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary heterodyne speckle imaging system is represented in FIG. 2.Such an exemplary heterodyne speckle imaging sensor system 200 is showncomprising a laser source 201; a beam splitter 202; 750 mm bi-convexlens 203; a first mirror 204; a dynamic diffuse scatterer 205; 500 mmbi-convex lens 206; a second mirror 207; 250 mm plano-convex lens 208; athird mirror 209; a polarizing filter 210; a first acousto-opticmodulator AOM1 211, 80.01 MHz upshift; a second acousto-optic modulatorAOM2 (212), 80 MHz downshift; a fourth mirror 213; 50 mm plano-convexlens 214; 50 μm diameter spatial filter 215; 250 mm plano-convex lens216; another polarizing filter 217; another beam splitter 218; 64×64pixel detector 219; a trans impedance amplifier 220 sampling at 30 kHz;and a signal processor and data display 221.

FIG. 3 shows an exemplary method for simultaneous observation ofthree-degrees of vibrational freedom 300 using a heterodyne beam, e.g.,of a heterodyne speckle imaging sensor system shown in FIG. 2. Referringnow to FIGS. 2 and 3, the source 201 is a linearly polarized laser 301.A splitter 202 divides the source into a probe and reference beams 302.In the probe beam path, a single lens provides focus adjustment for 303the beam 203 and mirror 204 redirects the beam to a dynamic object 205;the object has a diffuse surface. The electric field scatters 305 fromthe moving object 205 and now has a random spatial phase or amplitude.The purpose of the invention is to determine changes to the surface'sposition by tracking changes in the random, scattered electric field. Asthe object is tilted and translated, a phase shift is imparted due tothe optical path change near the object plane 205. Thus, the opticalfield at the object is modified by a phase associated with the newposition of the object.

$\begin{matrix}\left. {u_{O}\left( {\xi,\eta} \right)}\rightarrow{{u_{O}\left( {\xi,\eta} \right)}{\mathbb{e}}^{\frac{{j2\pi\Delta}\; P}{\lambda}}} \right. & (1)\end{matrix}$Where λ is the optical wavelength.

Some of the resulting scattered radiation is collected 306 by a singlelens 206. The radiation is redirected 307 using a mirror 207 andpropagates through a secondary optic 208 which defocuses 308 the fieldrelative to the image plane 219. An additional mirror 209 redirects thebeam 309 through a dichroic polarizing filter 210 where the horizontallypolarized radiation is absorbed 310. The radiation from the probe beampropagates to a second beam splitter 218 where half the radiationpropagates through the splitter and half is redirected towards the focalplane array 219. The probe-leg electric field at the image plane 219 canbe described as U_(P)(x,y,t₁), at one time, and U_(P)(x,y,t₂) at a latertime. Where U_(P)(x,y,t₂) is a phase modified version of U_(P)(x,y,t₁),the field at a previous frame. The relationship between the these fieldsis

$\begin{matrix}{{U_{p}\left( {x,y,t_{2}} \right)} = {{\mathbb{e}}^{\frac{{j4\pi}{({{\theta\xi}_{O} + d_{4}})}}{\lambda}}{\mathbb{e}}^{{- {j2}}\;{kx}\;\theta}{\mathbb{e}}^{j\; 2{kz}\;\theta^{2}}{U_{p}\left( {{x - {2\theta\; z}},{y - {2\phi\; z}},t_{1}} \right)}}} & (2)\end{matrix}$Where k is the optical wavenumber. The invention makes use of thisshifted and phase modified version of the original field to determinethe object's motion; the spatial shift corresponds to tilting of thesurface 205 while the phase corresponds to the axial motion of thesurface 205.

Following initial propagation through the beam splitter 202, thereference beam propagates through two acousto-optic modulators AOM1(211) and AOM2 (212). AOM1 211 upshifts the frequency of theelectromagnetic field 311 by a specified amount AOM2 212 downshift thefrequency 312 by an amount less than the upshift producing a modestfrequency offset of the reference field. A mirror 213 redirects thereference beam 313 where it propagates through a focusing lens 214 tofocus the beam 314. An aperture 215 is placed at the focal point of 214,acting as a spatial filter to spatial filter the beam 315. The divergingbeam propagates to another lens 216 which collimates the expanded beam316. The resulting electromagnetic radiation propagates through adichroic polarizing filter 217 where the horizontally polarizedradiation is absorbed 317. The reference field propagates to the beamsplitter 218 where half the radiation transmits through the splitter.

The device uses the same propagation distance, the distance betweensplitter mirror 202 and beam splitter 218, on both probe and referencefields. The beam splitter 218 additively combines the probe andreference electric fields 318 in the region between splitter 218 and thefocal plane array 219. The total field U_(t) at the image plane is thesuperposition of the probe and reference fields.U _(t)(x,y,t)=U _(r)(x,y,t)+U _(p)(x,y,t)  (3)The focal plane array 219 transduces the irradiance of these two fields319 into an electrical charge proportional to the irradiance.I(x,y,t)=U _(t) U _(t) *=U _(r) U _(r) *+U _(m) U _(m) *+U _(r) U _(m)*+U _(r) *U _(m)  (4)

$\begin{matrix}{{I\left( {x,y,t} \right)} = {\frac{1}{\lambda^{2}z^{2}}\left\lbrack {R^{2} + {P}^{2} + {2\; R{P}\cos\;\left( {{\omega_{LO}t} + {\frac{4\pi}{\lambda}\left( {{{\theta(t)}\xi_{0}} + {{\phi(t)}\eta_{0}} + {d_{4}(t)}} \right)} - \alpha_{p}} \right\rbrack}} \right.}} & (5)\end{matrix}$Where R and |P| are the magnitudes of the reference and probe fieldrespectively. The combined irradiance is the sum of three terms: 1)irradiance of the reference R², 2) irradiance of the probe |P|² and 3)the mix term R|P|.

The system relies on a heterodyne component where the reference fieldstrength amplifies the probe field. This is possible since the referencefield is frequency shifted at a frequency equal to ω_(LO), thedifference frequency of AOM1 211 and AOM2 212. The magnitude 2R|P| ofthe heterodyne irradiance shifts spatially in direct proportion to thesurface tilt 205. The phase terms associated with cos(ω_(LO)t+ψ) containsufficient information to compute the z-axis velocity of the surface.This phase term modulates the local oscillator frequency, ω_(LO). Thephase is

$\begin{matrix}{{\psi(t)} = {{\frac{4\pi}{\lambda}\left( {{{\theta(t)}\xi_{O}} + {{\phi(t)}\eta_{O}} + {d_{4}(t)}} \right)} - \alpha_{p}}} & (6)\end{matrix}$where the term containing d₄ is the Doppler shift due to translation ofthe object in the z dimension. The terms containing θ and φ are Dopplershift due to the tilting motion. It occurs in this expression becausethe pivots do not intersect the origin and the illumination is assumedto be centered at the origin. In other words, this term representsmotion in the z direction based on the amount of tilt and the relativedistance between the pivot point and the center of illumination. The twoterms just described do not depend on spatial position on the imageplane. In contrast α_(p)(x,y) is the phase of the probe speckle patternand will shift in a manner similar to the magnitude of the specklepattern.

A trans-impedance amplifier circuit 220 converts charge from the focalplane array 219 into a digital representation 320 of the irradiance on acomputer 221. The acquisition is repeated sequentially in time at aframe rate sufficient to observe the intermediate carrier frequencyresulting from the upshift and downshift from the acousto-opticmodulators 211 and 212. The focal plane array also has sufficient pixelsto observe speckles caused by light reflected from a diffuse target.

The sequence of image frames captured on the computer 221 undergoes aprocess on the same computer 321 to calculate tilt in the x and ydirections, Δθ and Δφ respectively, as well as axial velocity in the zdirection. The process is outlined in FIG. 4, with two processsequences.

FIG. 4 shows an exemplary process 400 for signal processing a sensorobservation of three-degrees of vibrational freedom using a heterodynebeam. The process to produce two dimensions of tilt is based onsequential estimates of the probe leg's speckle pattern amplitude.First, the image data stored on the computer 221 is accessed 401. Theheterodyne signal is isolated from the complete measurement by high-passfiltering 402 each pixel independently. This filter removes R² and |P|²from the camera's representation of the optical field, equation 5. Thefilter 402 outputs the heterodyne information at the local oscillatorfrequency ω_(LO). A temporal Hilbert transform 403 extracts the envelope2R|P| at each pixel. The output of 403 is a speckle pattern that shiftsspatially for each subsequent frame. Each frame is cross-correlated 404with its prior time frame producing a sequence of new data frames eachcontaining a spatial peak. A peak's position relative to center of theframe corresponds to the number of pixels shifted between two imageframes. A peak's location is estimated using a two dimensional parabolicspatial fit 405 on a cross correlation frame. In this way the peaklocation can capture sub-pixel shifts in the image pairs andconsequently small amplitudes of tilt. The output of the parabolic fit405 is a temporal sequence of peak locations applied in two dimensions,in other words, two temporal sequences of spatial shifts s_(x) and s_(y)between subsequent image frames. The spatial shifts are converted 406 tounits of object tilt difference Δθ and Δφ between subsequent imageframes,Δθ=s _(x)/2z  (7a)Δφ=s _(y)/2z.  (7b)The tilt can then be displayed as two time sequences on a data displaydevice 407.

A parallel sequence of the processing is used to calculate z-axisvelocity. This demodulation process uses a standard arctangentdemodulation for Doppler shifted heterodyne signals applied to eachpixel. (See, e.g., B. K. Park, O. Boric-Lubecke, and V. M. Lubecke,“Arctangent demodulation with DC offset compensation in quadratureDoppler radar receiver systems,” IEEE Trans. Microw. Theory Tech., vol.55, no. 5, pp. 1073-1079, May 2007, incorporated herein by reference.)The output of filter 402 provides the heterodyne information at thelocal oscillator frequency ω_(LO) necessary to begin the demodulation.In-phase and quadrature 408 are calculated by multiplying each pixel bythe sine and cosine of the local oscillator frequency ω_(LO). Theresulting in-phase I_(demod) and quadrature Q_(demod) time sequences arelow pass filtered 409 to remove unwanted components at frequenciesgreater than ω_(LO).Q _(demod)(t)=LP[HP[I(t)] sin(ω_(LO) t)]  (8)I _(demod)(t)=LP[HP[I(t)] cos(ω_(LO) t)]  (9)The in-phase and quadrature terms are then processed in block 410 thatcontains mathematical equation 10 and produces an estimate of theDoppler phase ψ at each pixel

$\begin{matrix}{\psi \approx {{{unwrap}\left\lbrack {\tan^{- 1}\left( \frac{Q_{demod}(t)}{I_{demod}(t)} \right)} \right\rbrack}.}} & (10)\end{matrix}$The velocity for each pixel is then estimated 411 using the Dopplerphase.

$\begin{matrix}{v = {\frac{\lambda}{4\pi}{\left( \frac{\psi_{2} - \psi_{1}}{t_{2} - t_{1}} \right).}}} & (11)\end{matrix}$The optical component 208 produces defocused images, as a result,estimates of velocity at each pixel should be equal. However,destructive interference from the diffuse scattering source 205 producessome low amplitude field strengths from the probe that are notaccurately captured with the camera 219 and 220. The velocity estimatesassociated with low amplitude pixels will not produce accurate estimatesof velocity. Process step 412 removes low signal strength pixels. Theremaining pixels are averaged 413 to reduce noise. The output isdisplayed 414 as a single velocity time series representing axialvelocity in the z direction.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed is:
 1. A heterodyne speckle imaging sensor system,comprising: a laser source emitting a linearly polarized laser; a firstbeam splitter to divide the linearly polarized laser from the lasersource into a probe beam along a probe path and a reference beam along areference path; a bi-convex probe lens in the probe path to providefocus adjustment for the probe beam; a first mirror in the probe path toreflect the probe beam as a redirected probe beam; a scattering sourcehaving a diffuse surface to scatter the redirected probe beam as ascattered radiation; bi-convex collecting lens to collect a portion ofthe scattered radiation as a collected radiation; a second mirror forredirecting the collected radiation as a propagated beam; plano-convexdefocusing lens to defocus a field of the propagated beam; a thirdmirror to redirect the resulting defocused propagated beam; a polarizingfilter to absorb horizontally polarized components of said propagatingbeam as a probe electric field towards an image plane; an arrangement ofacousto-optic modulators to effect frequency offset of theelectromagnetic field associated with the reference beam to result in afrequency offset reference beam; a reference mirror to redirect saidfrequency offset reference beam; a plano-convex reference lens to focussaid redirected frequency offset reference beam as a focused referencebeam; a reference spatial filter to spatial filter said focusedreference beam to result in a diverging beam; a plano-convex collimatinglens to collimate said diverging beam as a collimated reference beam;another polarizing filter to absorb horizontally polarized components ofsaid collimated reference beam as a reference electric field towardssaid image plane; a second beam splitter to additively combine the probeand reference electric fields towards said image plane; and a focalplane array disposed along said image plane, wherein said probe andreference electric fields as additively combined are sensed by saidfocal plane array for detection of changes in the random scatteredradiation.
 2. The heterodyne speckle imaging sensor system recited inclaim 1, wherein said arrangement of acousto-optic modulators iscomprised of: a first acousto-optic modulator to upshift a frequency ofthe electromagnetic field associated with the reference beam; and asecond acousto-optic modulator to effect downshift to the upshiftedfrequency of the electromagnetic field associated with the referencebeam by an amount less than the upshift to result in said frequencyoffset reference beam.
 3. The heterodyne speckle imaging sensor systemrecited in claim 1, wherein said focal plane array is a 64×64 pixeldetector.
 4. The heterodyne speckle imaging sensor system recited inclaim 1, further comprising: a trans impedance amplifier to convertcharge from the focal plane array into a digital representation ofirradiance as a detection output; and a signal processor to receive saiddetection output and compute random changes in the scattered radiationfor display output.
 5. The heterodyne speckle imaging sensor systemrecited in claim 1, wherein the focal plane array transduces irradianceof said probe and reference electric fields into an electrical chargeproportional to the irradiance as a measurable magnitude of heterodyneirradiance.
 6. The heterodyne speckle imaging sensor system recited inclaim 5, wherein the magnitude of heterodyne irradiance shifts spatiallyin direct proportion to a surface tilt of the dynamic scattering source.7. A method for simultaneous observation of three-degrees of vibrationalfreedom based on a heterodyne speckle imaging sensor system, said methodcomprising the steps of: emitting a linearly polarized laser from alaser source; dividing by a first beam splitter the linearly polarizedlaser from the laser source into a probe beam and a reference beam;providing focus adjustment for the probe beam through a first lens;redirecting the probe beam by a first mirror towards an objectdynamically disposed with respect to an object plane; scattering as ascattered radiation the redirected probe beam from a diffuse surface ofthe dynamic object moving in reference to the object plane, whereby anelectric field resulting from the scattered radiation has a randomspatial phase and amplitude; collecting a portion of the resultingscattered radiation through a collecting lens; redirecting the collectedportion of the resulting scattered radiation using a second mirror;propagating the redirected collected portion as a propagated beamthrough a secondary optic which defocuses the resulting propagated beamrelative to an image plane; redirecting the defocused propagated beamusing an additional mirror through a dichroic polarizing probe filterwhere a horizontally polarized radiation is absorbed, resulting in apolarizer filtered beam; splitting the polarizer filtered beam using asecond beam splitter, wherein a split portion of a radiation of thepolarizer filtered beam is reflected from a first surface of the secondbeam splitter as a reflected filtered probe beam towards a focal planearray disposed along said image plane; following initial propagationthrough the first beam splitter, effecting frequency offset of thereference beam to result in a frequency offset reference beam;redirecting said frequency offset reference beam using a reference beammirror to propagate toward and focus through a reference beam focusinglens to result in a focused reference beam; spatial filtering saidfocused reference beam using an aperture disposed at a focal point,resulting in a diverging beam; collimating the diverging beam using acollimating lens; filtering said collimated beam using a dichroicpolarizing reference filter where a horizontally polarized radiation isabsorbed and a filtered reference beam is transmitted; and directing thefiltered reference beam towards a second surface of the second beamsplitter, wherein a transmit portion of a radiation of the filteredreference beam is transmitted out the first surface of the second beamsplitter as a transmitted filtered reference beam towards said focalplane array disposed along said image plane, whereby said reflectedfiltered probe beam and said transmitted filtered reference beam asadditively combined allow focal plane array detection of changes in therandom scattered radiation.
 8. The method for simultaneous observationof three-degrees of vibrational freedom as recited in claim 7, whereinabout half of a radiation of the polarizer filtered beam propagatesthrough the first surface of the second splitter and a remaining portionof a radiation of the polarizer filtered beam is reflected from thefirst surface as the reflected filtered probe beam.
 9. The method forsimultaneous observation of three-degrees of vibrational freedom asrecited in claim 7, wherein said effecting frequency offset of thereference beam is comprised of the steps of: upshifting a frequency ofthe electromagnetic field associated with the reference beam using afirst acousto-optic modulator; and downshifting the upshifted frequencyof the electromagnetic field associated with the reference beam using asecond acousto-optic modulator by an amount less than the upshift toresult in the frequency offset reference beam.
 10. The method forsimultaneous observation of three-degrees of vibrational freedom asrecited in claim 7, wherein tilting and translating of said movingobject imparts a phase shift due to an optical path change near theobject plane, whereby a spatial shift corresponds to tilting of thediffuse surface of the dynamic object moving while the phase correspondsto an axial motion of the diffuse surface of the dynamic object moving.